Diastole.
Diastole is the recurrent and rhythmic relaxation or dilatation of the atria (upper heart chambers) and ventricles (lower heart chambers) of the heart during the cardiac cycle. This occurs in sequence and not together – that means the diastole of the atria alternates with that of the ventricles.
Usually, diastole is referred to as ventricular diastole (especially left ventricular). The heart chambers or cavities go into diastole to get filled with blood. Diastole is followed by systole in the cardiac cycle.
Ventricular Diastole.
During the ventricular systole, the ventricles pump the blood into the arteries – the aorta on the left side and the pulmonary artery on the right side. The ventricular systole is followed by the ventricular diastole during which the ventricular pressure drops below that of the atria, which have filled with blood coming from the vena cavae on the right and the pulmonary veins on the left side.
The pressure in the atria is therefore more than that in the ventricles. The AV valves therefore open and the blood flows from the atria into the ventricles. The ventricular diastole is indicated by the ‘T’ wave on ECG and has a duration of 0.16 seconds.
Atrial Diastole.
After the atrial systole, the atria are now devoid of blood. They go into diastole (they dilate) and get filled with blood coming from the vena cavae on the right side of the heart and the pulmonary veins on the left side of the heart.
Atrial diastole is followed by atrial systole. Similarly ventricular diastole is followed by ventricular systole. Both atria and the ventricles go into systole and diastole in unison.
The atrial diastole is usually marked by the QRS complex on ECG and has a duration of 0.08 seconds.
Diastolic Pressure in the Arteries.
This is the pressure in the aorta and the pulmonary artery during the ventricular diastole.
In the aorta, the pressure of blood during ventricular diastole is 80 mm of Hg while it is only 10 mm of Hg. in the pulmonary artery. These are the pressures indicated in the normal heart of a healthy adult.
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